(1) Field of the Invention
This invention relates to an aqueous matte coating composition.
(2) Description of the Background Art
An aqueous coating composition has widely been used, because it uses water as a medium and is free of troubles about working atmosphere, danger of fire, and so forth. However, recent diversification in user""s need shows a tendency to lose interest in feeling of metallic luster and highly demands matte coating film which provides grave feeling.
Particularly, user""s needs in recent years highly demand to provide an aqueous matte coating composition capable of forming a coating film showing difficulties in forming mars during transportation and showing high mar resistance.
Japanese Patent Publication No.24519/87 discloses a process for forming a matte coating film by use of an anionic electrodeposition coating composition comprising an alkoxysilyl group-containing vinyl copolymer as a base resin and an amino resin as a curing agent.
The aqueous matte coating composition used in the above process is such that the acrylic copolymer used as the base resin has alkoxysilyl group on its side chain and the alkoxysilyl group is gradually hydrolized on water solubilization or making water dispersion to form silanol, and further condensation between the silanols forms siloxane linkage, resulting in forming fine dispersion particles having an interparticle gel structure. As the result, when the fine dispersion particles are subjected to anionic electrodeposition coating, a film having a fine coarse surface is formed and even if heat-cured, the film keeps the coarse surface without forming a complete melt flow to form a good matte coating film, resulting in being widely used in the art.
However, use of the above aqueous matte coating composition makes it impossible to satisfy user""s needs of providing a coating film having high mar resistance, too. On the one hand, attempts to improve the mar resistance have been made, for example, by heat curing the coating film at high temperatures. On the other hand, developments of materials capable of drastically improve mar resistance have been demanded from the standpoint of the coating composition.
It is an object of the present invention to provide an aqueous matte coating composition which is capable of forming a matte coating film by use of conventional coating methods.
It is another object of the present invention to provide an aqueous mar-resistant matte coating composition which is capable of showing good storage stability, and capable of forming a coating film showing improved properties in mar resistance and durability.
That is, the present invention in a first embodiment provides an aqueous matte coating composition containing an aqueous dispersion (I) prepared by dispersing into water (B) a water-dispersible vinyl copolymer prepared by subjecting a monomer mixture of radically polymerizable unsaturated monomers including an unsaturated monomer having a polymerizable unsaturated double bond and an alkoxysilyl group in one molecule to radical polymerization in the presence of (A) a silicate compound consisting of an organosilicate represented by the general formula (1): 
where R is same or different and is hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carton atoms, and/or its condensate, having an alkoxysilyl group, an acid value of 15 to 150 mg KOH/g and a hydroxyl value of 30 to 200 mg KOH/g, and including the silicate compound (A), and (C) a melamine resin not having complete compatibility with the vinyl copolymer (B).
The present invention in a second embodiment provides an aqueous matte coating composition containing an aqueous dispersion (II) prepared by dispersing into water (A) a silicate compound as claimed in claim 1, (D) a water-dispersible vinyl copolymer having an alkoxysilyl group, an acid value of 15 to 150 mg KOH/g and a hydroxyl value of 30 to 200 mg KOH/g, and (C) a melamine resin not having complete compatibility with the vinyl copolymer (D).
The silicate compound (A) used in the first and second embodiments of the present invention includes ones represented by the above general formula (1), in which R is same or different and is hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms and its condensates. Use of such an organosilicate as to have a carbon number more than 10 in R results in reducing a speed of hydrolysis and in making poor the pollution resistance and durability of the film.
Examples of the monovalent hydrocarbon group having 1 to 10 carbon atoms in the above general formula may include alkyl group, aryl group and the like.
The above xe2x80x9calkyl groupxe2x80x9d may include straight-chain ones and branched ones, for example, methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, neopentyl, n-hexyl, iso-hexyl, n-octyl and the like. Of these, lower alkyl groups having 1 to 4 carbon atoms are preferred. The xe2x80x9caryl groupxe2x80x9d may include monocyclic and polycyclic ones, for example, phenyl group, toluyl group, xylyl group, naphtyl group and the like, phenyl group being preferred.
Preferable examples of the organosilicate used in the present invention may include tetrahydroxysilane, tetrame thoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, tetraphenoxysilane, dimethoxydiethoxysilane and the like. These may be used alone or in combination.
The condensate of organosilicate may include branched or straight-chain condensates between organosilicates represented by the above generai formula (1) and preferably includes condensates having a degree of condensation of 2 to 100 and represented by the general formula (2). 
where R is defined as above, n is an integer of 2 to 100, preferably 2-8.
In the above general formula, when n is more than 100, effect of pollution resistance is undesirably reduced.
The organosilicate used in the present invention may preferably include such ones that R in the general formula is a lower alkyl group having 1 to 4 carbon atoms, and the condensate thereof may particularly include such ones that R in the general formula is a lower alkyl group having 1 to 4 carbon atoms and that the degree of condensation is in the range of 2 to 15 preferably 2-8.
The first embodiment of the present invention is explained more in detail hereinafter.
The aqueous matte coating composition in the first embodiment of the present invention consists of an aqueous dispersion (I) containing a water-dispersible vinyl copolymer (B) and a melamine resin (C). The aqueous dispersion (I) is prepared by a process which comprises subjecting a monomer mixture of radically polymerizable unsaturated monomers including an unsaturated monomer having a polymerizable unsaturated double bond and an alkoxysilyl group in one molecule to radical polymerization in the presence of the silicate compound (A) to prepare a water-dispersible vinyl copolymer (B) having an alkoxysilyl group, an acid value of 15 to 150 mg KOH/g and a hydroxyl value of 30 to 200 mg KOH/g, and including the silicate compound (A), followed by adding (C) a melamine resin not having complete compatibility with the vinyl copolymer (B) prior to or after adding a neutralizing agent, adding a neutralizing agent if no neutralizing agent is previously added, and by dispersing into water.
An advantageous process for preparing the water-dispersible vinyl copolymer (B) having alkoxysilyl group is a method of copolymerizing a monomer mixture of an alkoxysilyl group-containing unsaturated monomer with other monomers. The above monomer mixture may include (1) an unsaturated monomer having a polymerizable unsaturated double bond and an alkoxysilyl group in one molecule, (2) xcex1, xcex2-ethylenically unsaturated carboxylic acid, (3) a hydroxyl group-containing acrylic monomer, and (4) a radically polymerizable unsaturated monomer other than the above monomers (1), (2) and (3).
The unsaturated monomer (1) having the polymerizable unsaturated double bond and alkoxysilyl group is a monomer component by which the alkoxysilyl group is introduced into the copolymer, includes, for example, unsaturated di- or trialkoxy (or alkoxyalkoxy) silane compounds such as divinyl dimethoxysilane (CH2xe2x95x90CH)2Si(OCH3)2, divinyl di-xcex2-methoxyethoxysilane (CH2xe2x95x90CH)2Si(OCH2CH2OCH3)2, vinyltrimethoxysilane CH2xe2x95x90CHSi(OCH3)3, vinyltriethoxysilane CH2xe2x95x90CHSi(OC2H5)3, vinyltris-xcex2-methoxyethoxysilane CH2xe2x95x90CHSi(OCH2CH2OCH3)3, xcex3-methacryloxypropyltrimethoxysilane, xcex3-methacryloxypropyltriethoxysilane, 
and the like. Of these, unsaturated trialkoxysilane compounds are preferred. This monomer may be used alone or in combination.
The alkoxysilane compound as the monomer component (1) is used in an amount of 0.1 to 10%, preferably 0.5 to 7% on the basis of a total weight (hereinafter on the same basis) of all the monomers. When the copolymer is water-dispersed, the alkoxysilyl group causes to form a polysiloxane linkage, resulting in increasing the molecular weight of the vinyl copolymer. When an amount of the alkoxysilane compound used is less than 0.1%, the above increase in molecular weight is not achieved with the result that a matte coating film having a low specular gloss is not obtained. On the other hand, when more than 10%, size of dispersed particles is so increased that sedimentation of the dispersed particles takes place and a non-uniform film is formed.
Examples of the xcex1,xcex2-ethylenically unsaturated carboxylic acid as the monomer component (2) include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, and the like. By use of the monomer component (2), carboxyl group is introduced into the vinyl copolymer, resulting in that the copolymer is water-dispersed by neutralization. The monomer component (2) may be used alone or in combination.
The xcex1,xcex2-ethylenically unsaturated carboxylic acid may be used in such an amount that the copolymer has an acid value of 15 to 150, preferably 20 to 100. When the acid value of the copolymer is less than 15, there is a tendency to make it difficult to form an aqueous dispersion. On the other hand, when the acid value is more than 150, water resistance of the film obtained may be reduced.
Examples of the hydroxyl group-containing acrylic monomer as the monomer component (3) include 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate and the like. By use of the above monomer, hydroxyl group is introduced into the acrylic copolymer to react with melamine resin for crosslinking reaction. The above monomer may be used alone or in combination.
The hydroxyl group-containing acrylic monomer is used in such an amount that the hydroxyl value is 30 to 200, preferably 50 to 150. When the hydroxyl value of the copolymer is less than 30, it shows poor film performance. On the other hand, when the hydroxyl value is more than 200, the film shows poor water resistance.
Other radically polymerizable unsaturated monomer as the monomer component (4) is a remaining component among monomer components for the vinyl copolymer, and may include known monomers usually used for the synthesis of vinyl copolymer, for example, C1-10 alkyl esters of acrylic acid or methacrylic acid such as methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, hexyl acrylate, octyl acrylate, lauryl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, butyl methacrylate, hexyl methacrylate, octyl methacrylate, lauryl methacrylate and the like; vinyl aromatic monomer such as styrene, xcex1-methylstyrene, vinyltoluene and the like; amide compounds of acrylic acid or methacrylic acid; acrylonitrile, methacrylonitrile, and the like. The above unsaturated monomer may be selected depending on the intended physical properties, and may be used alone or in combination.
Copolymerization of the above unsaturated monomers (1) to (4) may be carried out according to the known processes per se for the preparation of vinyl copolymer, for example, by use of solution polymerization process, emulsion polymerization process, suspension polymerization process, and the like. Advantageously, it is preferred to carry out according to the solution polymerization process, and it may be carried out by reacting the above four monomer components for about 1 to about 20 hours, preferably about 4 to about 10 hours at a reaction temperature of normally about 0xc2x0 C. to about 180xc2x0 C., preferably about 40xc2x0 C. to about 170xc2x0 C. in the presence of a polymerization catalyst in a suitable inactive solvent.
As the above solvent, it is desirable to use a solvent which is capable of dissolving the copolymer being produced and is miscible with water so that gelation may not take place during copolymerization reaction. Examples of such a solvent used may include cellosolve solvent, carbitol solvent, glyme solvent, cellosolve acetate solvent, alcoholic solvent and the like.
Examples of the polymerization catalyst used include azo compounds, peroxide compounds, sulfides, sulfines, diazo compounds, nitroso compounds and the like.
In addition to the above processes, the vinyl copolymer having alkoxysilyl group on its side chain may also be prepared by addition reaction of an epoxysilane compound such as xcex3-glycidoxypropyltrimethoxysilane or of an isocyanatosilane compound such as xcex3-isocyanatopropyltrietho xysilane to a vinyl copolymer synthesized beforehand and having carboxy group and hydroxyl group.
The vinyl copolymer (B) thus obtained has an acid value of 15 to 150 mg KOH/g, a hydroxyl value of 30 to 200 mg KOH/g and a number average molecular weight of about 10,000 to 100,000, preferably about 20,000 to about 60,000. When the number average molecular weight is less than 10,000, the film formed may not show satisfactory durability. On the other hand, when the number average molecular weight is more than 100,000, viscosity of the resin is so increased that formation of uniform fine particles is made difficult on being water-dispersed.
Formation of an aqueous dispersion of the vinyl copolymer (B) may be effected by the conventional method, for example, by neutralizing a vinyl copolymer containing alkoxysilyl group, hydroxyl group and carboxyl group as above obtained with amine compounds, for example, aliphatic amines such as monoethylamine, diethylamine, triethylamine and the like, alkanol amines such as diethanolamine, triethanolamine and the like, cycllic amines such as pyridine, piperidine and the like, and ammonia in an amount of 0.5 to 1.0 equivalent relative to the carboxyl group.
The melamine resin (C) used as a crosslinking agent for the water dispersible vinyl copolymer (B) in the present invention is required to be compatible by the aid of a solvent common to both, but not to have complete compatibility in the absence of any solvent, with the vinyl copolymer (B). It may be defined hereinbelow that the melamine resin does not have complete compatibility with the vinyl copolymer (B).
The vinyl copolymer (B) is formulated with melamine resin (C) in a solids weight ratio of (B)/(C) to be 40/60, an amine compound is then added in an amount of 1.0 equivalent relative to carboxyl group of the vinyl copolymer (B), and water is then added to form an aqueous dispersion having a solid content of 20% by weight. The aqueous dispersion is coated onto a transparent glass plate to a thickness of 10 xcexcm as a dry film, followed by removing the solvent by evaporation at room temperature to 100xc2x0C., and further by drying at a temperature of 150xc2x0 C. to 200xc2x0 C. for 5 to 10 minutes. When the film thus formed is found hazy by the naked eye, it is defined that the melamine resin (C) does not have complete compatibility with the vinyl copolymer (B). Further, quantitatively speaking, it may be defined that the melamine resin (C) does not have complete compatibility with the vinyl copolymer (B) when a percentage transmittance measured on the film obtained as above at a wavelength of 4000 xc3x85 by use of a spectrophotometer is 95% or less.
The melamine resin (C) used in the present invention has the percentage transmittance of 95% or less, preferably 90% to 50%, and has a weight average molecular weight of 400 to 4000, preferably 600 to 2500. The weight average molecular weight of the melamine resin represents one calculated in terms of polystyrene in gel permeation chromatography.
When the percentage transmittance is more than 95%, compatibility of the melamine resin with the vinyl copolymer is so increased that it is made impossible to form a matte film having a gloss of 50 or less (i.e. 60xc2x0 specular reflection, and so forth).
The melamine resin (C) used in the present invention is not specifically limited so long as the above percentage transmittance is satisfied, and may be used in the form of various etherified ones such as ones modified by use of at least one of methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, octyl alcohol, 2-ethylhexyl alcohol, benzyl alcohol and the like. Those modified with alcohols having preferably C3 or higher, more preferably C4 to C10 are preferred in the present invention. It is advantageous that the melamine resin contains an ether group modified by C3 or higher alcohol in an amount of, on the average, at least one, preferably 2 to 4 per one nucleus of triazine ring.
Mixing amounts of respective components (A), (B) and (C) are explained hereinafter.
A mixing ratio of the vinyl copolymer (B) to the melamine resin (C) are such that (B)/(C) is in the range of 95/5 to 30/70, preferably 90/10 to 40/60 based on the weight of solid contents.
A mixing amount of the silicate compound (A) is such that a mixing ratio of the silicate compound (A) to a total amount of the vinyl copolymer (B) and the melamine resin (C) is in the range of 5:95 to 70:30, preferably 10:90 to 50:50 on the basis of the weight of solid contents.
The aqueous matte coating composition in the second embodiment of the present invention consists of an aqueous dispersion (II) containing the silicate compound (A), a water-dispersible vinyl copolymer (D) and the melamine resin (C). The aqueous dispersion (II) is prepared by a process which comprises adding the silicate compound (A) and the melamine resin (C) not having complete compatibility with the vinyl copolymer (D) prior to or after adding a neutralizing agent, followed by adding a neutralizing agent if no neutralizing agent has been added, and by dispersing into water.
The vinyl copolymer (D) may be prepared by subjecting a monomer mixture of the above monomers (1) to (4) to radical polymerization in the same manner as in the first embodiment of the present invention, except that the radical polymerization is carried out in the absence of the silicate compound (A).
The vinyl copolymer (D) has an acid value of 15 to 150 mg KOH/g, a hydroxyl value of 30 to 200 mg KOH/g, and a number average molecular weight in the range of about 10,000 to 100,000, preferably about 20,000 to about 60,000. When the number average molecular weight is less than 10,000, the film formed may tnot show satisfactory durability. On the other hand, when the number average molecular weight is more than 100,000, viscosity of the resin is so increased that formation of uniform fine particles is made difficult on being water-dispersed.
The silicate compound (A) and the melamine resin (C) used in the second embodiment of the present invention are the same as those used in the first embodiment of the present invention.
A mixing ratio of the vinyl copolymer (D) to the melamine resin (C) is in the range of 95/5 to 30/70, preferably 90/10 to 40/60 in terms of the weight of the solid content.
A mixing amount of the silicate compound (A) is such that a mixing ratio of the silicate compound (A) to a total amount of the vinyl copolymer (D) and the melamine resin (C) is in the range of 5/95 to 70/30, preferably 10/90 to 50/50.
As required, pigments, dyes and additives may be added to the aqueous matte coating composition of the present invention.
The aqueous matte coating composition of the present invention may be coated onto various kinds of metal materials such as aluminum, aluminum alloy, anodized aluminum, steel; a steel sheet plated with zinc, tin, chromium, aluminum and the like; a steel sheet subjected to a chemical treatment with chromic acid or phosphoric acid, or to a cathodic electrochemical treatment; and the like. Further, the aqueous matte coating composition may be coated directly onto the metal material, or, as a topcoating, onto a film formed by coating the known undercoating and intermediate coating.
The aqueous matte coating composition of the present invention may be coated onto the surface of the above metal material by means of, for example, spray coating, electrostatic spray.coating, brushing, dip coating, roll coating, electrodeposition coating and the like. The coated film thickness is normally about 5 to 100 xcexcm, preferably about 10 to 80 xcexcm. After coating, curing at about 100to 200xc2x0 C. for about 10 to 60 minutes forms a matte film.
When the electrodeposition coating is employed as the coating method, it is usually carried out under the conditions of a coating bath temperature of 15 to 35xc2x0 C., a coating voltage of 80 to 350 V and a treating time of 1 to 5 minutes. After the completion of the electrodeposition coating, without needing a step of washing with water, heat-curing forms an intended matte film.
The effects of the present invention are explained below.
It is guessed from the following reasons that the aqueous matte coating composition of the present invention is capble of forming a matte film showing high mar resistance by means of any known coating methods other than the electrodeposition coating, too.
An irregular reflection on the surface due to the formation of roughness on the surface of the film is essentially necessary for the formation of a matte film, no matter what coating method may be used, either the electrodeposition coating method or other coating methods, and further, in the case of a clear film, an irregular reflection within the film layer also acts supplementally thereto to lower its gloss.
The use of the aqueous matte coating composition of the present invention results in forming a matte film in that the incomplet compatibility of the melamine resin with the vinyl copolymer causes to produce a localization of the melamine resin within the film layer, resulting in producing irregular reflection within the film layer, and in that the formation of fine roughness due to the separation and localization of the vinyl copolymer and the melamine resin in the surface area of the film is fixed by the melt flow inhabitation caused by the formation of gel particles due to the alkoxysilyl group when heat cured.
On the other hand, high crosslink density of the coating film due to the presence of the organosilicate and migration of the organosilicate to the surface layer of the coating film may cause to increase hardness on the surface of the coating film, resulting in providing improved film properties in mar resistance.